The spine principally includes a series of vertebrae and spinal discs located in a space between adjacent vertebrae. The vertebrae are formed of hard bone while the discs comprise a comparatively soft annulus and nucleus. The discs support the vertebrae in proper position and enable the torso to be rotated and to bend laterally and anteriorly-posteriorly. The discs also act as shock absorbers or cushions when the spine is experiencing shock, such as during running or jumping.
A variety of spinal conditions result in a person experiencing pain or limited physical activity and ability. More specifically, damage to vertebrae composing the spine and spinal discs between the vertebrae may occur as a result of trauma, deformity, disease, or other degenerative conditions. Some of these conditions can be life-threatening, while others cause impingement on the spinal cord resulting in pain and a lack of mobility. Removing the impingement, thus reducing swelling or pressure from the damaged or diseased tissue against the spinal cord, can relieve the pain and often promotes healing and return of normal nervous system functioning. However, the absence of proper medical care may lead to further damage and degeneration of spinal health and to permanent spinal cord damage.
Damage to the spine often results in reduced physiological capability. For instance, damage to the disc or vertebra may allow the annulus to bulge, commonly referred to as a herniated disc. In more severe cases, the damage may allow the nucleus to leak from the annulus. In any event, such damage often causes the vertebrae to shift closer or compress, and often causes a portion of the disc to press against the spinal cord.
One manner of treating these conditions is through immobilization of the vertebrae in a portion of the spine, such as two or more adjacent vertebrae, which is often beneficial in reducing or eliminating pain. Immobilization and/or fusion have been performed via a number of techniques and devices, and the type of injury often suggests a preferred treatment regime.
One form of immobilization is known as spinal fusion surgery, in which two or more adjacent or consecutive vertebrae are initially immobilized relative to each other and, over time, become fused in a desired spatial relationship. For instance, rigid rods or plates may be attached to the spine to immobilize the spine for a sufficient length of time to allow fusion between the vertebrae and the intervertebral implant to take place and/or to allow boney ingrowth into the disc space. Desirably the vertebrae are relatively immobilized at the proper intervertebral distance, replicating the support characteristics of the healthy spine. An implant spacer may be placed between the vertebrae, which over time will fuse to both adjacent vertebrae or be entirely resorbed and replaced with bony growth.
Spinal fusion surgery substantially reduces or eliminates the motion between vertebral segments, which thereby alleviates a source of pain in some patients. Although fusion treatments sacrifice rotation and flexion between the affected vertebrae such that some loss of movement and flexibility of the spine is experienced, the non-fused portions of the spine are largely able to compensate for most normal movement expected by a patient. Furthermore, the compression on the spinal cord due to the injury is reduced or eliminated, and the fused vertebrae protect the spinal cord from injury. The spinal fusion surgery can also be used to prevent or impede progressive deformity of the spine in some patients. Subjects in need of spinal fusion include those suffering from fractured vertebra, spondylolisthesis, protrusion or degeneration of the spinal disc, abnormal curvature of the spine (such as scoliosis or kyphosis), spinal tumor, spinal infection, or spinal instability.
Currently, a number of fusion devices are known. Implantation of such devices may involve excavating a portion of one or both adjacent vertebrae to provide a volume for locating the device therein. Many fusion devices have surface features such as anchor members in the form of prongs, teeth, spikes, and the like, which extend away from upper and lower surfaces of the device for being embedded into the adjacent vertebrae. In order to locate the device within the intervertebral space, instruments may be used to spread the vertebrae apart.
Some fusion devices are made from bio-resorbable materials, such as natural bone, hydroxyapatite, calcium phosphates, and other bone-like compositions Fusion devices comprising bio-resorbable materials can be integrated into adjacent bone over time eventually providing a single solid mass of bone, however devices prepared using such materials tend to have relatively weak compressive strengths.
Stronger materials, such as polyetheretherketones (PEEK), may be used to manufacture fusion devices to provide increased strength. Surface features such as spikes and teeth made of such materials can penetrate vertebral bone to secure the device thereto. However, these stronger materials are not bio-resorbable.
The purpose of the fusion procedure is to develop a lattice, matrix, or solid mass of bone joined with and extending between the adjacent vertebrae and through the intervertebral space. Eventually, the formed or developed bone and the vertebrae are joined to provide a somewhat unitary, incompressible structure that maintains the proper pre-fusion spatial relationship for the size to reduce or eliminate the impingement on the spinal cord. Accordingly implants formed of these stronger materials are left as a redundant structure which is unable to be absorbed by the body or replaced by bone growth and which acts as a boundary interface between the implant device and any resultant bone growth. While the effect of the boundary interface can be addressed by reducing the size of the implant so that more graft material can be packed into the intervertebral space around the implant, this can result in less secure implantation.
Accordingly, there is a need for improved spinal fusion systems and for improved methods for performing spinal fusion surgery. There is also a need for implants that are compatible with body chemistry and physiology and that possess mechanical stability for hardness, compressive strength, flexural strength, and/or wear resistance, as well as controlled microstructure to develop functional gradients, controlled interfacial properties to maintain structural integrity in physiological conditions, and/or surface chemistry tailored to provide appropriate adhesion properties, chemical resistance, and lasting patient comfort.
The present invention may be used to fulfill these, as well as other needs and objectives, as will be apparent from the following description of embodiments of the present invention.